In the production of a cup-shaped article made of tin plate, aluminum or other material by deep drawing, a blank is formed into a shallow cup-shaped article, and then the said cup shaped article is drawn into a deep cup-shaped article by stretching the wall. At this time the distribution of the wall thickness along the length is not uniform.
The force to remove or strip the thus formed cup-shaped article from a punch varies according to the amount of reduction of the wall thickness, the forming speed, the distribution of the wall thickness, the height of the cup-shaped article and so on. The said force becomes large when the cup-shaped article is formed of steel plate.
The usual method of stripping the cup-shaped article by using a knock out ring or a hook to hold the open end of the cup-shaped article while the punch is withdrawn often causes a deformation of the wall at the open end part, as shown in FIG. 2. The less the thickness of the wall, the more likely this is to occur. The wall thickness of a cup-shaped article made of steel is less than one of aluminium, and therefore it is even more difficult to strip a cup-shaped steel article from the punch by using the knock out ring only.
To overcome this problem, there has been developed a method using an internal knock out pin provided inside of the punch in order to drive the bottom of the cup-shaped article away from the end of the punch, as disclosed, for example, in U.S. Pat. Nos. 3,270,544 and 3,390,565. However, the mechanism for forming the cup-shaped article which includes such a knock-out pin is complex.
Moreover, the forming speed is limited by the internal knock out pin which moves faster than the punch and by the shock on the article accompanying the action of the pin. Another disadvantage of the internal knock out pin is that the shape of the bottom of the cup shaped article is limited due to the presence of the internal knock out pin which moves in and out of the end of the punch.
Another common practice has been to taper the punch used to form cup-shaped articles so that it has a smaller diameter at the punch end in order to be able to easily strip the formed cup from the punch. However, cans for beer or the like beverages are necked in and flanged at the open end after the cup-shaped can is formed, so that it is necessary for these cans to have a thicker wall at the top or open end so as to be able to withstand such deformation. Therefore it is almost impossible, in making a punch to form these cans, to taper the punch to make it easy to strip the formed cup-shaped can therefrom.
The less the wall thickness of a can, the more economical it is to make. However, the top end part of the wall must have a minimum thickness in order to withstand the working thereof during the neck-in and flanging operation. The diameter of the punch corresponding to the top end part of the can must therefore be smaller than the remainder of the punch if the lower wall of the finished cup has a smaller thickness than the top end part of the wall. This makes stripping the cup-shaped article from the punch even more difficult.